Gadolinium enriched in its odd numbered atomic mass isotopes, Gd.sup.157 and Gd.sup.155, is of commercial interest, particularly for use as burnable poisons in light water nuclear power reactors. In particular, gadolinium enriched in the Gd.sup.157 isotope is useful in fuel cycle design. It would therefore be desirable to enrich gadolinium in its 157 and 155 isotopes and in particular in its 157 isotope.
Enrichment of an atomic vapor in one or more isotopes may be achieved through separation processes which operate on very small differences between the chemical or physical properties of the isotopes, such as by selectively exciting energy states of one or more isotopes (without corresponding excitation and ionization of the other isotopes) whereby the selectively ionized isotopes may be separated from the neutral atoms by an electrical field. However, designing a scheme for selective ionization of the desired isotope in an efficient manner, including not only the atomic transitions utilized in the isolation scheme, but also the particular sources of photon energy required for overall efficiency of the process, are not readily apparent. The discovery, cataloging and design of the particular energy levels for a photoionization scheme are critical in determining the proper pathways which will result in an advantageous, efficient and commercially usable photoionization scheme.